In the field of packaging, plants are known which are used to perform a plurality of creasings, or segments of preferential folding, on a packaging material, for example cardboard, so as to facilitate and guide the folding of the sheet, to define a packaging box.
In general, these plants are suitable to continuously receive a strip of the material, and are provided with creasing units able to perform on the strip transverse and longitudinal creasings distanced from each other by a determinate pitch.
The creasing units can also be suitable to cut the strip to size, to define individual sheets. The sheets correspond in size to the development of the box to be made.
Document GB 2 323 566 A describes a creasing device of a known type, to make folding grooves on both faces of a piece of cardboard.
Two types of creasing are substantially known, respectively blind or continuous, and through or segmented.
Continuous creasing provides a substantially local compression on the thickness of the material along an ideal folding line of the sheet, while segmented creasing provides to make alternating through notches, or through cut segments, along the folding line.
The choice to perform one type of creasing or the other depends on the specifications of the material, the creasing or other.
In any case, a generic creasing device is typically provided with a creasing disk that is made to rotate around an axis, which may be transverse to the direction in which the sheet to be worked is fed, and acts along the same direction of feed in order to achieve a longitudinal creasing, or to rotate around an axis parallel to the direction of feed, and acts transverse to it in order to achieve a transverse creasing.
One disadvantage of known creasing devices is found particularly in working corrugated cardboard and especially in small-size machines, that is, for small productions, but with great flexibility in working different formats.
In order to contain costs, weight and bulk, such small-size machines comprise small creasing discs. It should also be considered, however, that in industrial machines the diameter of a creasing disc can reach as much as 600/700/900/1000 mm, and therefore a very great weight, especially if multiplied by the usually large number of creasing discs that an industrial machine provides.
In small-size machines, especially when working corrugated cardboard which has the so-called ridges adjacent to each other, providing small creasing discs causes the material or ridges to “explode” along the creasing line, inasmuch as, given the same creasing force applied, the pressure is locally much greater and applied violently, instantaneously and substantially locally on the thickness of the material, that is, on the ridge. Indeed, the pressure acts on the internal chambers of the corrugated cardboard which are compressed and, depending on the thickness of the individual internal corrugated layers, resist to a greater or lesser extent against the deformation and, beyond a certain limit, cause the material that surrounds them to explode and break. The more the thickness increases, the more amplified this effect is, for example for double corrugation cardboard, and the more the thickness of the internal layers is reduced. This is so much greater in creasing operations performed transverse to the main direction of development of the corrugations or ridges. Typically, the cardboard is fed with the corrugations transverse to the work direction, also called machine direction, and hence this disadvantage mainly occurs in longitudinal creasing operations, that is, where the axis of rotation of the creasing discs is transverse to the work direction. In industrial machines, which have much bigger costs, bulk and weight, this phenomenon is limited because large creasing discs are able to perform a progressive creasing and to prevent, or at least reduce, the explosion of the material.
Purpose of the present invention is to obviate the disadvantages of the state of the art and to obtain a creasing device, and perfect a creasing method, particularly for small machines with great working flexibility, which allows to achieve a progressive working of the sheet fed, preventing the phenomenon of exploding material, especially in the case of sheets of corrugated cardboard.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.